(1). Field of the Invention.
The present invention relates to the field of semiconductor manufacturing, and more specifically to the formation of contact holes for a dual Damascene structure.
(2). Description of the Prior Art.
The present invention relates specifically to the Damascene process that is used for the formation of semiconductor devices. Damascene derives its name from the ancient art involving inlaying metal in ceramic or wood for decorative purposes. In Very Large-Scale Integrated circuit applications, the Damascene process refers to a similar structure.
The Damascene process has been demonstrated on a number of applications. The most commonly applied process is first metal or local interconnects. Some early Damascene structures have been achieved using Reactive Ion Etching (RIE) but Chemical Mechanical Planarization (CMP) is used exclusively today. Metal interconnects using Damascene of copper and of aluminum is also being explored.
FIG. 1a gives an overview of the steps of the Damascene process, as follows:
Step 1 shows the formation of the metal plug, PA1 step 2 shows the deposition of the Inter Level Dielectric, PA1 step 3 shows the formation of the trenches for metal lines, PA1 step 4 shows the deposition of metal to fill the trenches, PA1 step 5 shows the removal of metal from the surface.
The Damascene process is further explained below, the numbers indicated within this explanation refer to the cross section of a Damascene structure that is shown in FIG.1b.
Referring now specifically to FIG. 1a there is shown the formation of a metal via plug 10 within the semi-conductor substrate 14 (FIG. 1e). Any micro-scratch present on the surface will fill with metal during subsequent metal deposition and can cause electrical shorts between adjacent via plugs 10 or between electrical lines deposited on top of surface 12. To remove the Damascene residue and to remove the scratch count on the surface 12, surface 12 is polished and buffed after the metal plugs 10 have been deposited.
FIG. 1b shows the deposition of the Intra-Level Dielectric (ILD) 16 (FIG. 1b) which can be deposited using Plasma Enhanced CVD (PECVD) technology. Dielectric 16 can, for instance, be SiO.sub.2.
FIG. 1c shows the formation of the trenches 18 for the metal lines, these trenches 18 can be formed using Reactive Ion Etching (RIE) technology.
FIG. 1d shows the deposition of metal 20 to fill the trenches, this process can use either the CVD or a metal flow process. The excess metal on the surface is removed using the CMP process, see FIG. 1e and a planar structure 26 with metal inlays 22 in the intra-level dielectric 16 is achieved.
The application of the Damascene process continues to gain wider acceptance, most notably in the process of copper metalization due to the difficulty of copper dry etch where the Damascene plug penetrates deep in very small, sub-half micron, Ultra Large Scale integrated devices. Recent applications have successfully used copper as a conducting metal line, most notably in the construct of CMOS 6-layer copper metal devices. Even for these applications however, the wolfram plug was still used for contact points in order to avoid damage to the devices.
FIG. 2a shows Prior Art problems encountered when filling a Damascene Wolfram plug 62 with aluminum 64. The plug 62 can be formed in poly silicide 66. A void 60 can develop above the opening of a Damascene plug 62 if the opening is relatively narrow and deep, a design characteristic that becomes more common with smaller semiconductor devices. This void 60 is caused by the difficulty experienced in having deep penetrating flow of the Al within the narrow opening. For a shallow or relatively wide plug 62, FIG. 2b, these problems are not experienced. Void 60 (FIG. 2a) also causes planarization problems during subsequent processing steps and can create a reliability issue.
FIG. 3 shows a Prior Art blanket deposition of metal within the hole 30. This hole 30 can exists in a semiconductor substrate 36. Where the hole 30 is relatively shallow and wide, no problems of deposition are experienced, see FIG. 3a. This blanket deposition requires polish back, in the absence of polish back problems of shorts between metal lines arises caused by remaining metal on top of the surface. FIG. 3b demonstrates another Prior Art approach where the top 32 of the plug 34 is further extended by overfill. The extension 32 can be obtained by depositing a layer of wolfram across the surface and applying an etchback to that layer such that wolfram is left in place around the top of the plug.
An extension of the damascene process is the dual damascene process whereby an insulating or dielectric material, such as silicon oxide, is patterned with several thousand openings for the conductive lines and vias, which are filled at the same time with metal. Damascene is an interconnection fabrication process in which grooves are formed in an insulating layer and filled with metal to form the conductive lines. Dual damascene is a multi-level interconnection process in which, in-addition to forming the grooves of single damascene, conductive via openings also are formed. One of the dual damascene approaches uses a dielectric layer that is formed by three consecutive depositions whereby the central layer functions as an etch stop layer. This etch stop layer can be SiN, the top and bottom layer of this three layer configuration can be SiO.sub.2. This triple layer dielectric allows first forming the vias by resist patterning the vias and etching through the three layers of dielectric. The conductive pattern can then be formed in the top layer of dielectric whereby the central layer of SiN forms the stop layer for the etch of the conducting pattern. Another approach, still using the three-layer dielectric formed on the substrate surface, is to first form the pattern for the conducting lines in the top layer of the dielectric whereby the SIN layer again serves as etch stop. The vias can then be formed by aligning the via pattern with the pattern of the conducting lines and patterning and etching the vias through the etch stop layer of SiN and the first layer of dielectric. Yet another approach is to deposit the three layer dielectric in two steps, first depositing the first layer of SiO.sub.2 and the etch stop layer of SiN. At this point the via pattern can be exposed and etched. The top layer of SiO.sub.2 dielectric is then deposited; the conducting lines are now patterned and etched. The SiN layer will stop the etching except where the via openings have already been etched.
Yet another approach to forming the dual damascene structure is to form an insulating layer that is coated with a photoresist. The photoresist is exposed through a first mask with image pattern of the via openings, this via pattern is anisotropically etched in the upper half of the insulating layer. The photoresist now is exposed through a second mask with an image pattern of the conductive line. The pattern of the conducting lines is aligned with the pattern of the vias thereby encompassing the via openings. In anisotropically etching the openings for the conductive lines in the upper half of the insulating material, the via openings already present in the upper half are simultaneously etched and replicated in the lower half of the insulating material.
Dual damascene is an improvement over single damascene because it permits the filling of both the conductive grooves and vias with metal at the same time, thereby eliminating process steps.
In short, Prior Art experiences problems in creating a plug for the Damascene process that provides a reliable connect. In filling deep or narrow holes, problems of aluminum voids can arise. This in turn causes problems with planarization of subsequent layers that are deposited over the Damascene plug since these layers may now be deposited on a surface of poor planarity.
In overfilling a shallow hole, a polish-back is required in order to avoid shorts by leftover materials between metal lines. Polish-back further complicates the process and adds to the expense incurred while in many instances polishing has to be done in combination with buffing in order to obtain acceptable planarization.
U.S. Pat. No. 5,564,245 (Allen) shows a Cu interconnect over a W plug with a barrier layer therebetween.
U.S. Pat. No. 5,744,376 (Chan et al.) discloses a Cu interconnect formed using V*IN barrier layer and Damascene process.
U.S. Pat. No. 5,770,517 (Gardner et al.) shows Cu interconnect formed using another barrier layer and Damascene process.
U.S. Pat. No. 5,612,254 (Mu et al.) shows a dual Damascene structure using Cu and a barrier layer. However, this reference differs from the present invention.